Search for Charged Lepton Flavor Violation at BESIII

TL;DR

The paper investigates charged lepton flavor violation (CLFV) in charmonium decays using BESIII's world-leading datasets. It searches for $J/\psi\to e\tau$, $J/\psi\to e\mu$, and $\psi(3686)\to e\mu$ decays with dedicated reconstruction, background suppression, and a profile-likelihood framework to set upper limits. No CLFV signal is observed, and the resulting branching-fraction limits are $\mathcal{B}(J/\psi\to e\tau)<7.5\times10^{-8}$, $\mathcal{B}(J/\psi\to e\mu)<4.5\times10^{-9}$, and $\mathcal{B}(\psi(3686)\to e^{\pm}\mu^{\mp})<1.4\times10^{-8}$ at 90% C.L. These limits are interpreted in an effective-field-theory context to constrain Wilson coefficients $|C_{DL}^{\ell_{1}\ell_{2}}/\Lambda^2|$, $|C_{DR}^{\ell_{1}\ell_{2}}/\Lambda^2|$, $|C_{VL}^{q\ell_{1}\ell_{2}}/\Lambda^2|$, $|C_{VR}^{q\ell_{1}\ell_{2}}/\Lambda^2|$, $|C_{TL}^{q\ell_{1}\ell_{2}}/\Lambda^2|$, and $|C_{TR}^{q\ell_{1}\ell_{2}}/\Lambda^2|$, thereby probing NP scales beyond direct reach. The results exemplify BESIII’s capability to test CLFV and guide NP model-building.

Abstract

Charged lepton flavor violation (CLFV) is forbidden in the Standard Model but predicted by many new physics models. We present searches for CLFV in charmonium decays using world-leading datasets collected by the BESIII detector. The processes $J/ψ\to eτ$, $J/ψ\to eμ$, and $ψ(3686)\to eμ$ are investigated using world-leading $J/ψ$ and $ψ(3686)$ dataset collected by BESIII. No significant signals are observed, and upper limits on branching fractions are set at $\mathcal{B}(J/ψ\to eτ)<7.5\times10^{-8}$, $\mathcal{B}(J/ψ\to eμ)<4.5\times10^{-9}$, and $\mathcal{B}(ψ(3686)\to eμ)<1.4\times10^{-8}$ at 90\% confidence level. These results provide constraints on Wilson coefficients in effective field theory and probe new physics at high energy scales.

Figures (1)

• Figure 1: (Left, Central, Right) Likelihood distribution as a function of the branching fraction of CLFV decays $J/\psi\to e\mu$, $J/\psi\to e\mu$, and $\psi(3686)\to e\mu$, respectively. The arrow indicates the upper limit at 90% C.L.